1. Field of the Invention
The present disclosure relates to coating systems f, and more particularly to coating systems for gas turbine engine components, for example.
2. Description of Related Art
Traditionally, the durability and the maximum temperature capability of a thermal barrier coating (TBC) system used in gas turbine engines is often limited by deposits of naturally occurring calcium-magnesium-alumino-silicate (CMAS). These deposits melt and wet the material, typically yttria-stabilized zirconia, used as the thermal barrier coating, causing it to be drawn by capillarity into all of the open void space. Upon cooling, when the CMAS solidifies, the penetrated layer develops a high modulus of elasticity. Since the thermal barrier coatings rely on spatially configured voids to achieve strain tolerance with the superalloy substrate, those regions penetrated by the CMAS can be detrimental, causing the thermal barrier coating to be susceptible to extensive spallation when subjected to subsequent thermal cycles. Thermal barrier coating spallation can lead to a considerable reduction in the gas turbine engine component durability and, if not addressed, to a potential direct attack on the underlying substrate.
A traditional approach to deal with this problem is to deposit an extra layer over the thermal barrier coating. However, this extra layer is not activated prior to the introduction of the component into service. As a result, it does not become activated until CMAS is encountered in service. CMAS comes with a variety of chemical compositions depending upon its geographical origin. Consequently, the effectiveness of this extra layer is unknown and may be less than desirable in certain conditions.
One approach is to deposit a reactive layer with known CMAS reaction kinetics onto the thermal barrier coating and apply a heat treatment to the reactive layer prior to using the component in service, for example, a twenty-four hour heat treatment between 2100° F. and 2200° F. (1149° C. and 1204° C.). Examples of such systems and methods are disclosed in U.S. Patent Publication No. 2012/0034491 which is incorporated by reference herein in its entirety. Such reactive layer coating and heat treatment can mitigate the effects of spallation due to CMAS.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for coating systems that allow for improved CMAS resistance.